Group-delay equalizer using a meander folded transmission line

ABSTRACT

A group-delay equalizer for microwave frequencies, comprising a meander folded transmission line, comprising means for separately changing the electric wave lengths of said folded transmission line associated with two different modes of propagation of the electromagnetic energy along said line.

United States Patent l l Caroli [45] Aug. 19, 1975 l l GROUP-DELAYEQUALIZER USING A MEANDER FOLDED TRANSMISSION LINE [75] Inventor:[gnazio Caroli. Rome Ital I73] Assignec Seleniz-lndustrie ElettronicheAssociate S.p.A., Italy [III Filed: Apr. 4, [974 [Ill Appl, No; 457.825

|3U| Foreign Application Priority Data Apr 13. 1973 Ital v r r r v .44J4f1l/7] ISZI LBS. 333/28 R; 333/3l R; 333/84 M ISI] Int. Cl .0 Hlllp9/00; H03h 7/l6 (58} Field of Search 333/28 R. 3] R. 84 M |S6|References Cited O'lHl-IR PUBLICATIONS Hewitt. A Computer Designed. 720to l Microwave Compression Filter IEEE, Trans. on MTT. December 1967, p.687-694 TK7SOOIZ3.

lrl'mur tliruminer-Paul L. Gensler Almrncy. Agent, or l-irmHilL Gross.Simpson Van Santen. Steadman, Chiara 6: Simpson 1571 ABSTRACT Agroup-delay equalizer for microwave frequencies comprising a meanderfolded transmission line com prising means for separately changing theelectric wave lengths of said folded transmission line associated withtwo different modes of propagation of the electromagnetic energy alongsaid line.

2 Claims. l8 Drawing Figures PATENTEU AUG T7. 900 8 O6 SL- IU 1 BF 6cuaanc'rems'nc :aeauencv Foe \m-ncn C i THE LINE as 5 LONG PATENIEU mm 9I975 Ab i// GROUP-DELAY EQUALIZER USING A MEANDER FOLDED TRANSMISSIONLINE The present invention relates to an improvement in the group-delayequalizers for micro-wave frequencies.

More particularly, the present invention relates to devices forequalizing the group-delay characteristics introduced by transmissionmedia, such as wave guides or radio-frequency filters in circuits.

Depending upon the specific utilizations, there is a remarkablevariation in the necessary operating parameters. In slopes or linearcomponents:

I. Group-delay introduced by transmissive media in dispersive guide.Typical application: Transmission of messages with digital modulation,with desired bandwidths of (SOD-I000) MHz and group-delay variations upto the order of some tens of nanoseconds.

2. Group-delay, introduced by branching circuits for the oscillationswhich between the input port and the output port are reflected bycircuits (generally passband filters) showing a reactive impedancevariable with the frequency. Typical application: Branching circuits inthe radio relays or repeaters with desired bandwidths in the order of 25MHz and variations of the group-delay from about to about 10nanoseconds. In Parabolic component:

Group-delay introduced by RF filters and RF amplifiers. The applicationscan be: wide band l()%) or narrow band l%). In each case, the variationof group-delay is typically in the range of nanoseconds.

The typical field of utilization of the device according to thisinvention is therefore as follows:

a. as a component in transmission systems with millimeter waves forequalizing wide band groupdelays;

b. as a component for equalizing group-delays in connection withmicro-wave components in frequency bands usually not greater than (-6)GHz.

In the prior art, problems of the related kind were solved at leastpartially with wave guides having pro gressively variablecross-sectional areas in connection with circulators or hybrids. withthe possible addition of adjusting means consisting of a movable sheetof dielectric.

Other known systems provide for the use of circulators or hybrids inconnection with reactive circuits. One known solution is that proposedby the U.S. Pat. No. 3,277,403 of S. B. Cohn, entitled Microwave dualmode resonator apparatus for equalizing non linear phase angle of othercomponents of October 4, 1966.

A further solution of the problem has been proposed in the literature byJones and Bolljahn (E. M. T. Jones, J. T. Bolljahn; "Coupled StripTransmission Filters I.R.E. Microwave, April I956). In this publication,reference is made to an all pass structure with a line folded betweentwo ground planes.

The present invention aims to solve the problem of equalizing thecharacteristics of group-delay in terms of the frequency with a deviceof the all pass structure with a folded line capable of operating withwide band with reduced overall dimensions, and adjustable for permittingthe necessary flexibility of use of the device and for compensating themanufacture tolcr anccs which can become severe at the openingfrequencies. The solution according to this invention allows also toobtain with reduced overall dimensions said equalization of group-delaywith narrow band 1I2) at relatively low frequencies 4GHz) where the useof dual mode circularly polarized cavity reactance elements would implyexcessively large mechanical overall dimensions (see the above mentionedU.S. Pat. No. 3,227,403).

The present invention will be now described with reference to oneembodiment thereof described by way of example, and with reference tothe attached drawings, wherein:

FIG. I shows the base unit of a meander folded line for introducing thesymbols used in the specification;

FIG. 2 diagrammatically shows the static capacitances correlated to twopropagation modes (odd and even respectively) for lines coupled to eachother and located between two ground planes;

FIGS. 3 and 4 are diagrams showing the trend of the group-delay versusthe electric lengths of the transmission line;

FIG. 5 shows diagrammatically a sectional view of a folded line betweentwo ground planes; I

FIGS. 6 and 7 show further views of folded lines;

FIG. 8 shows a first sectional view taken along the plane A-B of FIG. 9of an embodiment of the equalizer according to this invention of theuniform line type;

FIG. 9 shows a second sectional view according to the plane C-D of FIG.8;

FIG. 10 shows a perspective view of the module of dielectric materialused in the structure shown in FIGS. 8 and 9;

FIGS. 11a, b, c show examples of embodiment of the detail A shown inFIG. 10;

FIG. 12 shows a first sectional view taken along the line CD of FIG. I3of an embodiment of the equalizer according to this invention with alumped loading line;

FIG. 13 shows a second sectional view taken along the plane A-B of FIG.12;

FIGS. I4, 15 and 16 show respectively partial sectional views takenalong the planes E-F, G-H and l-L as shown in FIG. I2. 1

With reference now to the drawings, and particularly to FIG. I, a foldedline can be considered as a quadripole of two-port structure of the allpass type with image impedance corresponding to Z at all frequencies. Itis capable of introducing a group-delay variable with the frequencysince the phase of the oscillation between the input port and the outputport is a non-linear function of the electric length of the line.

The symbols as follows will be now introduced, useful for the subsequentspecification of this invention.

With reference to FIG. I, let us assume:

f,. the frequency for which is 0 )t/4 6 the electric length of the line;

Z Z the characteristic impedances associated with the two modes ofpropagation along the line, odd and even" respectively;

6 6 the electric lengths of the line associated to the two odd and evenmodes of propagation characteristic parameter or coupling coefficient;

A=(1+K)/(ll().

It can be demonstrated that when 6 6 6 we shall have tan

where r is the group delay. In FIG. 3 there has been shown the trend ofthe group delay r versus 6 expressed by the relation (l) for a given f}.

As the features of the structure of FIG. 1 arise from the directional"properties of two coupled lines, this structure can be used twice to sumthe contributions represented by the relation l relative to one elementwith a single folding. The checking of the values which appear in therelation l) leads to the result that by a suitable selection of saidvalues it will be possible to synthesize various characteristics of thegroup-delay versus the frequency.

The example of the above appears in the FIG. 3 wherein it can be notedthat it is possible to change the slope of the characteristic ofgroup-delay frequency introduced into the circuitry by changing thevalue taken by the 6 at the operative frequency, in other words bychanging the value of the characteristic frequency f Also the non linearcomponent or almost parabolic component can be adjusted by operating apart (for instance one-half) of the structure with determined values ofthe electric length 6. This possibility is shown in the diagram of FIG.4.

However, the fact that we must have 8 32 0,,,., as it appears from theliterature. and the fact that it is practically impossible to obtain amechanical adjustment of the length of the line, the adjustment of thedevice becomes a problem (besides the actuation for fixed frequencieswith acceptable tolerances), and therefore will limit the practicalembodiments to nonadjustable structures.

Notwithstanding the above, according to the present invention. theadjustment is still possible if a method is discovered for embodyingadjusting means capable of varying separately the two propagationvelocities relative to the even and odd modes, such as by a mechanicaladjustment of the dielectric structure connected to the folded line,and/or by the adjustment of lumped discontinuities embodied by metallicor dielectric elements, that is operating with non-uniform andnonhomogeneous lines.

In the following disclosure, by way of clearness but not for limitingpurposes, reference will be made to a non-homogeneous uniform line to ahomogeneous and non'uniform linc.

First case: uniform and non-homogeneous line Let us consider thestructure of FIG. 2 wherein have been diagrammatically shown two groundplanes, electrically conductive and denoted by the numeral referencesl0," spaced apart from one another through a distance b. Between theseground planes 10,11 is located in a substantially centered location thefolded line, consisting of a strip. shown in sectional view andreferenced as 12,l3,l4,l for an example of an indefinite repetitivestructure.

In the conductor sections l3, 14, the static capacitances have beenrespectively shown relative to the even and odd modes of propagationalong the line.

With reference to this Figure, it is therefore possible to observe thatthe partial or total presence of a dielectric in the zone I, as shown bya shadow and/or in the symmetrical zone, affects primarily thecharacteristic impedance Z and the velocity 1' relative to the odd mode,and the value of C 4C'f 2C,,, will be altered by the efiect on the valueof Cf On the contrary, the total or partial presence of dielectric inthe region 2 near the ground planes (shown in FIG. 2 by the samecriteria as the region 1) affects essentially the characteristicimpedance Z and the velocity V of the even mode, and the value of C 4C'f2C, will be altered. Also, the value of the capacitance C D will easilytake account of this fact when the unit is designed or adjusted. lnfact, however, in case of wlb l corresponding to the cases of practicalinterest, and for values of coupling between the lines like thosenecessary in the practical cases, the contribution of C will benegligible. Consequently, a variation of the amount of dielectricpresent in the above indicated regions (1 and 2) allows the values ofthe electric lengths of the lines associated to the two even and oddpropagation modes to be separately changed.

In the most general case, a practical embodiment of means for changingthe capacitive loading would render necessary the study of the electricfield between the two lines in the partial presence of dielectric. Forthe cases of practical interest, it is possible to proceed to a strictreal dimensioning of a structure partially filled with a dielectric, byusing the methods for the determi nation of the capacitances of C 'f,,and C f,, as appeared in the literature.

In connection therewith, let us consider the PK}. 5. In this Figure thestructure comprising the line folded between two ground planes issubstantially entirely filled with a dielectric. This structurecomprises the ground conductor planes A,B; the dielectric blocks Dl,D2and the conductor of the folded line denoted by Ll,L2,L3

The two blocks of dielectric material D1, D2 can be spaced apart fromone another along the plane aa. A variation of the distance b2, being bb b:, obtained by spacing away the conductor plane A and leaving fixedthe position of the upper face of the dielectric block D2, obtained byintroducing an air gap Ab between the conductor plane A and the uppersurface of the dielectric block D produces substantially a decrease ofthe capacitance value C 'f correspondingly, a variation of the distance11,, obtained by spacing away the dielectric block D in the interface ofaa and introducing an air gap A11 between the two dielectric blocks byspacing away the conductor plane B together with the dielectric block Dproduces substantially a decrease of the value of the capacitance C f,,.

Then, it is possible according to this invention to have two separateadjustments, practically embodied for Cf, and for ("f permitting theembodiment of a folded line transmission structure wherein it will bepossible to adjust the values 6 and 9 satisfying the condition 6, 6

As a particular case, considering the dielectric blocks D D ashomogeneous, and with a relative dielectric constant 6, E 2.3, we shallhave with reference to the symbols shown in FIG. 5 and for the casewhere w/b l and Ab z .3173 Ali. we shall have:

The contribution of (3) is substantially a parasitic effect of theadjustment which however can be corrected by the adjusting means. Theeffect of (4) is that of changing the impedance Z, of the structure,limited. however, to a practically allowable amount which can becompensated by acting in a way substantially analo gous to that aspreviously described, or with lumped constants on the line tractsconnecting to the load.

Assuming b mm, the percent variation of the characteristic frequency f,.which can be expected according to (2) is:

Hereinafter a possible practical embodiment of the device according tothis invention will be described by way of example.

Second Case: non-uniform line either immersed or not in homogeneousdielectric Still considering the diagrammatical representation of FIG. 2let us now have reference to FIGS. 6 and 7 showing the diagrammaticalrepresentation of a line with a single folding useful for understandingthe second embodiment of this invention concerning nonuniform lines,immersed or not immersed in a homogeneous dielectric.

The line comprises a U-shaped conductor located between two conductorground planes AB. A susceptance [7,. located in the regions R and Rallows the adjustment of the electric length 6, relative to the evenmode and a susceptance 12,, also located in the regions R and R allowsthe adjustment of the electric length 6, relative to the odd mode. Thepractical embodiment can be obtained by placing the conductor 20 formingthe line in a dielectric, either air or solid, and the susceptances band 1),, can be formed by metal screws or dielectric screws.

The possibility of adjusting independently the two susceptances allowsthe condition of equal electric length to be obtained for the modes ofpropagation. If h 12 I b, the distance 6 between the regions R R will be2!) 0=arc tan to which corresponds an electrical length between theregions R,, R equalling 0,, R 11 Therefore the percent variation of theelectrical length which is to be obtained being 1;, we have from whichwe obtain:

Now two practical embodiments of the group-delay equalizer for microwavefrequencies will be described.

EXAMPLE I Equalizer with uniform line Reference is made to FIGS. 8 toII. The device comprises a first plate 30 with at least the inwardlyturned surface conductive, and a second plate 31 similar to the firstcited plate. Between the plates 30, 31 a spacer 32 is interposed. To theplate 31 is fixed a module of dielectric material shaped as shown inFIG. I0 fixed to the plate 31 by the screws 36. A similar module 34 isfixed by screws 35 to the spacer 32.

The transmission line, meander folded, shown as a unit by 37, isinserted and fixed in the grooves 38 provided in the dielectric module33. Two tracts of line 39, 40 serve for the electrical connection to theconnecting terminals of the device to the balance of the device.

In FIGS. Ila, I lb and 114 there is shown the relationship between striptransmission line 37 and the fingers of the dielectric module 33, in thefolding zone. In FIG. I la the strip 37 follows a segmented fold and aportion of dielectric 33 is removed in the corners as shown. In FIG. 11bin the fold zone the strip 37 follows a curvilinear path and thedielectric 33 is correspondingly shaped. In FIG. Ilc, the strip 37 isbent as in FIG. Ila and the dielectric 33 has a corresponding shape butwithout empty spaces.

The adjustment of the characteristics of the equalizer is obtained byvarying the distance of the lower surface of the plate 30 with respectto the upper surface of the dielectric module 34 and changing theimmersion of the dielectric block or module 33 in the line by changingthe distance of the plate 31 with respect to the spacer 32 (see FIG. 5for reference to the previous disclosure).

The adjustment of the aforesaid distances can be obtained by thebushings 41, 41', 4] and 41'. The structure of these bushings and theassociated mechanisms will be described with reference to the bushing 41as both the other shown bushings and others corresponding by symmetryand not shown in the Figure, are identical. The bushing 41 in the shapeof an overturned cup, bears against the outer surface of the plate 30and is provided with an internal threading 42 which engages a threadedpin 43 forming the prolongation of a post 44 along which can slide, dueto the holes 46, the plate 30. A contrast spring 46 tends to space awaythe plate 30 from the spacer 32. The lower module 33 can have reduceddimensions with respect to the dimensions of the module 34 and if asingle adjustment of the constants will be sufficient, it can also beentirely dispensed with.

Of course, it is possible to provide for the connection in series and/orparallel of more equalizers according to this invention, as well as tomodify the mechanical structure of the unit in order to obtainindependent adjustments of the same tract of folded line, with alongitudinal division of the dielectric modules. Said alternatives areobviously capable of being designed by a person skilled in the art andwill not be described in detail.

EXAMPLE 2 Equalizer with line with sole lumped adjustments Reference isto be made to F168. 12 to 16. The device comprises a box type structuregenerally denoted by 47, which is conductive at least internally,including an upper plate 48, a lower plate 49 and a spacer 50. Insidethe box type structure 47, the folded line 51 is supported by littleblocks of low loss insulating material 52,53,54. The line tracts 55,56serve for connecting the equalizer to the electrical terminals for theconnection to the remainder of the circuit.

FIG. 12 shows in 57,58, adjusting susceptances in a possible embodimentthereof. In 59,60,61 said susceptances have been shown in a differentpossible embodi ment.

As it will be better noted in FIG. 13 the susceptances 57 include afirst screw element 62 which is threaded in the lower plate 49 and whichpresents toward the line a flat end, circular with a diameter greaterthan the maximum lateral dimension of the part of line on which itoperates, at the center of which is screw threaded a screw 63 which canextend inside the space between the lines. The elements 62 and 63 acttherefore also on the electric lengths ofthe folded line. FIGS.14.15.16. show partially interrupted sectional views of FIG. 12 showingthe geometrical relation between the adjusting elements 59, 60 operatingon the electrical lengths.

(iii

FIGS. 12 to 14 show the line of the equalizer device immersed in air.Said line could also be immersed into a homogeneous dielectric.

Furthermore, the lumped susceptances can also be practically made ofconductor material or if dielectric material.

Having thus described the present invention, what is claimed is:

l. A group-delay equalizer for microwave frequencies, comprising ameander folded transmission line, and adjusting means for changingseparately the electric lengths of said folded transmission lineassociated to two different modes of propagation of the electromagneticenergy along said line, wherein said folded line is located between twoground planes, said adjusting means comprising a first block ofsubstantially homogeneous dielectric material into which is immersed theaforesaid strip line partially folded lengthwise, a second block ofdielectric material substantially similar to said first dielectric blockof which it is possible to adjust the immersion of said line; means forchanging the distance of one of said ground planes with respect to theouter surface of said first dielectric block, and means for changing thedistance of the other of said ground planes to which is fixed saidsecond dielectric block in order to change the immersion of said seconddielectric block into said folded line.

2. A device as claimed in claim 1, characterized in that said blocks ofdielectric material comprise differ' ent amounts of dielectric in theactive zone.

1. A group-delay equalizer for microwave frequencies, comprising ameander folded transmission line, and adjusting means for changingseparately the electric lengths of said folded transmission lineassociated to two different modes of propagation of the electromagneticenergy along said line, wherein said folded line is located between twoground planes, said adjusting means comprising a first block ofsubstantially homogeneous dielectric material into which is immersed theaforesaid strip line partially folded lengthwise, a second block ofdielectric material substantially similar to said first dielectric blockof which it is possible to adjust the immersion of said line; means forchanging the distance of one of said ground planes with respect to theouteR surface of said first dielectric block, and means for changing thedistance of the other of said ground planes to which is fixed saidsecond dielectric block in order to change the immersion of said seconddielectric block into said folded line.
 2. A device as claimed in claim1, characterized in that said blocks of dielectric material comprisedifferent amounts of dielectric in the active zone.